Dot colour as seen through safety goggles

This has got me baffled. A laser produces monochromatic light, so unless it is being shone on a fluorescing surface, the reflected light should have the same wavelength as the beam.

Thus if you view the dot from a red laser through WL blue safety goggles, you ought to see a (dim) red dot. And indeed that is exactly what I do see.

So then I tried it with the dot from a green laser viewed through DL orange/red safety goggles. But I didn't see a dim green dot. What I actually saw was a yellow dot!

Hmmm I thought. What's going on? Then I figured out the answer. What I am actually seeing is a combination of the filtered green dot + some minor 808 leakage through the IR filter. Red+ green = yellow. Eureka! And I patted myself on the back.

Trouble is...when I viewed the dot from my 405 BR through the same DL orange/red safety goggles, the theory fell apart. Unlike the (DPSSL) green, the output from the 405 is pure 405. So as with the red dot, I ought to see a dim version of the original - i.e. 405 violet.

But lo and behold, what I actually saw was yellow (again)!!!

Please can somebody explain what is going on??!! What single theory explains why 650 through blue appears as 650 but both 532 DPSSL and 405 through orange/red both appear yellow?

Thanks.

David

It's because the pigment in the goggles is somewhat fluorescent.

Edit: And the reason the red still appears red through the blue goggles is because red does not stimulate fluorescence... at least in the visible spectrum.

Yes I guess that could explain it, but......

1. I have just tried looking at the 405 dot through a uranium glass vase. The vase flurosces a brilliant green colour when the 405 dot is shone on it, but when the dot is viewed on a wall through the vase, the dot appears violet, not green.

2. When the dot is shone on the goggles from the front, there is no sign of fluorescence and the dot reflects violet off the front of the goggles.

Just tried viewing 532 and 405 dots through red filter glass (which I bought to check for IR leakage).

Both dots appear red!!

Weird.

davidgdg said:

Yes I guess that could explain it, but......

1. I have just tried looking at the 405 dot through a uranium glass vase. The vase flurosces a brilliant green colour when the 405 dot is shone on it, but when the dot is viewed on a wall through the vase, the dot appears violet, not green.

2. When the dot is shone on the goggles from the front, there is no sign of fluorescence and the dot reflects violet off the front of the goggles.

Click to expand...

Fluorescence does not change the color of the beam. Fluorescence is when the absorption of a photon stimulates the emission of another photon at a lower energy state... or something like that.
So, in fluorescence, the photons emitted by the object that your laser is directed at will be of a longer wavelength (orange, yellow) than that of the original beam. This is why fluorescence from red lasers is not noticeable, because it will be in the deeper red and NIR part of the spectrum. Reflections from that object or the beam passing through it will still be the same color/wavelength.

The fluorescence in the goggles is probably not noticeable when the laser is shone on it because it is not brilliantly fluorescent like some other objects and the dot of the laser will overpower the tiny amount of fluorescence.

Goggles are meant to reflect and attenuate certain wavelengths so the amount that is absorbed to stimulate fluorescence will be minimal.

And the IR leakage from your green laser would not be nearly enough to combine with the green to make yellow.